During the course of the last decade, the problem of unwanted detonation of high energy explosives and propellants for weaponry and rockets has been a major concern to the military. In an effort to increase propellant/explosive survivability and reduce the threat of unplanned stimuli, such as the risk of unwanted detonation, various attempts have been made to incorporate elastomeric binders into the high energy explosive material to produce low vulnerability (so-called "LOVA") propellants/explosives. The binders studied include, for example, thermoplastic and thermosetting polyethers, acrylates and cellulose derivatives. General background information on the testing of these types of materials, as well as a general discourse on the desired binder characteristics for LOVA propellants is given in a technical paper by S. Wise and J. J. Rocchio entitled "Binder Requirements for Low Vulnerability Propellants", presented at the 18th JANNAF Combustion Meeting, Volume II at the Jet Propulsion Laboratory in Pasadena, California on October 19 to 23, 1981, and reproduced in CPIA publication 347, Chemical Propulsion Information Agency, pp. 305-319, (October, 1981).
Desired binder characteristics include thermal stability against decomposition at a high temperature of at least about 180.degree. C., processability by melt methods at temperatures below about 100.degree. C., a low glass transition temperature of less than about -65.degree. C., and the ability to depolymerize endothermically when exposed to the acidic decomposition products of energetic materials in propellants and explosives. In addition, there are several other preferred characteristics of binders, including the ability to maintain desired physical properties, including the above binder characteristics, even when loaded with up to 80 percent or higher of energetic additives, as well as the ability ideally to avoid phase changes over a temperature range of between about -60.degree. C. and about 60.degree. C., the ability to maintain binder integrity during elongation of up to 400 percent, and the ability to provide a high oxygen content and thereby assist in ignition of the explosive's or propellant's high energy component.
Recent technical papers on LOVA binders were presented at the "ONR Workshop on Energetic Polymer and Processing Science" given in Chestertown, Md. on July 25-27, 1983 and reproduced in CPIA Publication 405, Chemical Propulsion Information Agency (June, 1984). In a paper reproduced ar pages 131-139 of CPIA Publication 405, J. C. W. Chien generally describes the "Synthesis of Polyacetal-Polyurethane Thermoplastic Elastomers as LOVA Binders" by end capping methylene(bisphenylisocyanate (MDI) or toluene diisocyanate (TDI) to form a prepolymer, and then chain extending this prepolymer with a monomer which is either bis(2-hydroxyethyl) terephthalamide or bis(2-hydroxyethyl-isophthalamide). The testing of these types of polyurethane polyacetals formed by reacting an isocyanate-terminated prepolymer with a monomer is disclosed in an article by S. T. Peters et al entitled "Characterization of Thermoplastic Elastomers for Use in Gun Propellants" from Proceedings of the American Defense Preparedness Association; Chemical & Plastics Section of the Chemical Systems Division, held October 27-29, 1986, Long Beach, Calif., pp. 8-15. Table I at page 12 of this article discloses, in Samples 17 through 22 of the table, physical property data on these types of polymers. These samples show thermal stability, when loaded with 50 percent of RDX in a binder/propellant formulation, up to a decomposition temperature peak of between 191.degree. C. and 210.degree. C., as compared to the decomposition temperature of RDX alone of 220.degree. C. This difference in decomposition temperature between the pure RDX and the RDX/binder mixtures indicates some interaction between RDX and these binders, believed to be an undesirable effect.
This S. T. Peters et al article also discloses in TABLE I, samples 21 and 22, the testing of two polyacetal polyurethane elastomers of another type. This other type is apparently fabricated by reacting an isocyanate-terminated prepolymer, made using either TDI or MDI as the isocyanate component of the prepolymer, with a polyurethane and chain extended with N,N'(bis-2-hydroxyethyl) isophthalamide. These two samples are described in this article as exhibiting undesirable decomposition temperature peaks of 191.degree. C. and 195.degree. C., respectively, in admixture with a 50 percent loading of RDX. In addition, these two polymers are disclosed as having poor processability characteristics due to unacceptably high viscosities and high melting points.
In view of the above, it should be readily compositions containing improved polyurethane polyacetal elastomer binders and characterized by improved battlefield survivability and having desired mechanical properties, including a desired degree of hardness, would be highly desired by the military, particularly the propellant and explosives communities. Heretofore, such LOVA compositions and a method for their preparation were not known to the knowledge of the present inventor(s).